Building a gaming computer

I am trying to build a computer used for just gaming and is based off Windows 7 64-bit. What kinda hardware am I going to need to have no lag whatsoever? I have the shell of a 2004 Compaq persario. I'm willing to change specs if needed

Yeah, I really doubt that you could reuse anything from that comp, we need more information. What games you want to play? Are you a casual, hardcore, or enthusiast gamer? Do you want to build your comp, or have a boutique build it for you? WHAT IS YOUR BUDGET!!! (most important part) Your descriptions are extremely vague. What do you mean by "I'm willing to change the specs if needed" Do you just want to modify your old comp? If really want to truly game I would not reccomend that. Do asethetics matter? How much storage do you need? Do you want to run multiple monitors?

Last but not least, How much time do you want to spend on this? My last comp took 3 months to choose the perfect combination of parts and to read up on each of the reviews.

No better place to begin than this.
Firstly, we need to know your budget. High-end components will give no lag, but a low budget is limiting on how high-end you can get. You can get a complete review of popular components here.
The Presario's shell may only be good if

1) It matches your components (not colour, but size).​

2) It has excellent airflow, suited to your system.​

In any case, here are a few points:

Processor: You may choose what you like, but at the end of the day, only the best will do. And the best isn't the most expensive either. For a gaming rig, you options vary from Intel Ivy Bridge (yeah, I finally recommend it) to the Athlon II and Phenom II. Given Intel's superior performance as of now, I, an AMD fanboy, recommend you the Ivy Bridge. Native 64-bit CPUs, suited to your needs.

Motherboard: Not much to be said. Ivy Bridge chipset, Asrock or Gigabyte. I prefer Asrock. Better customer support. A review of best 4 is here. Go through it, and skip nothing. Just make sure the box has a "Compatible with Windows 7" logo. That means it's compatible with W7x64.

Storage: Often overlooked, the Hard-disks play a big role in gaming performance. Get hold a good SSD if you can. Or a high-RPM Seagate.

GPU: Nvidia Keplers. Or better still (IMBO), Radeon HD 7000 series.

CD-ROMs: Not that I like these, but I use a Sony. I never had a good experience with the 3 Samsungs that lie in my cupboard somewhere.

Monitor: An Acer, or ASUS. Depends on your budget and needs really, so suit to taste.

PSU: The stock ones that come with the case are usually not enough for gamers (usually they need 450W+). For that you need something like Corsair, or other 80+ certified PSUs. What wattage PSU you select depends on your system's (especially GPU's) needs.

Cooling systems: By this I mean either an after-market air-cooling system or water-cooling kit. For selection help for both air-cooling and water-cooling kits, here's an excellent place to begin.

Hope I covered everything. If I missed something, please do ask.
Good luck on your new rig! Cheers!